1. Field of the Invention
The present invention relates to a cathode for an electron tube and a method of preparing the same, and more particularly, to a cathode which is improved in life and electron emission characteristics by realizing the high density and surface evenness of an electron emissive layer for an electron tube.
2. Description of the Background Art
As a cathode for an electron tube, an oxide cathode is widely used. The oxide cathode includes an electron emissive material layer formed of oxides converted from an alkaline earth metal carbonate containing barium as a main component, and preferably, ternary carbonate having a basic composition of (Ba, Sr, Ca)CO3 or binary carbonate having a basic composition of (Ba, Sr)CO3 is converted, on a base metal containing nickel (Ni) as a main component and slight amounts of silicon (Si) and magnesium (Mg) as reducing agents. Since such an oxide cathode has a low work function, it has an advantage of operating at a relatively low temperature (700–800° C.). However, since the oxide cathode is formed of a semiconductor and has large electrical resistance, its raw material is evaporated or melted by self-heating due to Joule heat when electron emission density is increased, which thus deteriorates the cathode. Moreover, an intermediate resistance layer is formed between the metal base and the oxide layer due to prolonged operation, which shortens the life span of the cathode.
A typical cathode for an electron tube includes a base metal having the shape of a circular plate, a cylindrical sleeve which is provided below the base metal to support the base metal and has a heater, i.e., a heating source of the cathode, therewith, and an electron emissive material layer adhering to the top of the base metal.
The following description concerns a general procedure of preparing an oxide cathode and the principle of electron emission. Carbonate powder containing barium carbonate as a main component is mixed in an organic solvent in which nitrocellulose is dissolved. The resulting mixture is attached to a base metal using a spraying or electrodeposition method. The resulting structure is installed at an electron gun, which is provided in an electron tube. During evacuation for making the inside of the electron tube vacuous, the carbonate is heated to about 1000 degrees Celsius by a heater. Here, the barium carbonate is converted into barium oxide as follows.BaCO3→BaO+CO2  (1)
The barium oxide reacts with Si and Mg, which is a reductant contained in the base metal, at its boundary contacting the base metal during the operation of a cathode, and is reduced as the following chemical equations, thereby producing free barium. The barium contributes to the emission of electrons.BaO+Mg→MgO+Ba  (2)4BaO+Si→Ba2SiO4+2Ba  (3)
In preparing a conventional oxide cathode, a spraying method is most widely used as a method of attaching a mixture containing carbonate powder to a base metal. According to the spraying method, the powder of a raw material is dispersed into an organic solvent containing a binder to form a suspension, and the suspension is sprayed using the pressure of air stream flow, thereby accomplishing a coating. Nitrocellulose is used as a binder. Isoamylacetate, diethyloxalate, or pure water is used as a solvent.
However, since a spraying method uses only force sprayed by air pressure without using any other pressure, it is limited in obtaining a uniform and dense coating film. More specifically, the structure of an electron emissive material layer attached by a spraying method is shown in FIGS. 2 and 3. FIG. 2 is an electron microscopy photograph of the section, which is enlarged 200 times, of an electron emissive material layer attached by a spraying method. As shown in FIG. 2, the size of the pore between particles is nonuniform, the surface is very coarse, and the texture is sparse. FIG. 3 is an electron microscopy photograph of the surface texture, which is enlarged 2000 times, of the electron emissive material layer of FIG. 2. It can be confirmed again that the size of the pore between particles is nonuniform.
When a cathode with such an electron missive material layer having nonuniform particle size, pore size and smoothness is installed at an electron gun, defects occurs in a product, and the reliability in quality decreases. In addition, when the surface roughness is large due to the nonuniform particles size, a beam spot becomes smaller, which causes a mask to interfere with the beam spot during operation of a color cathode-ray tube, resulting in a moire phenomenon. Generally, in a cathode having a roughness of 20 micrometers or greater, a moire phenomenon occurs at a particular scan frequency. More specifically, the basic function of a cathode-ray tube is to control an electron beam emitted from a cathode. This function is affected by adjusting the voltage of an electron gun composed of the cathode and other electrodes (grids). Here, if the surface of the cathode is not smooth, a difference between a voltage applied to the cathode and a voltage applied to a G1 (first grid) electrode occurs, thereby making it hard to demonstrate normal characteristics. In addition, when the cathode-ray tube is driven for a long time, sintering of the cathode progresses. Here, if the texture of the cathode is not dense, pores initially formed are collapsed and shrunk, increasing the distance between the cathode and the G1 (first grid) electrode. Consequently, the potential difference, which is set to control an emitted electron beam, between the cathode and the G1 (first grid) electrode changes, which causes deterioration of a life characteristic and degradation of brightness due to decreases in the amount of a charge emitted. On the contrary, when the electron emissive material layer of a cathode is excessively compressed, the high density of the electron emissive material layer increases a probability of the electron emissive material layer being detached from a base metal. In addition, the range of conditions of decomposing and aging (activating) the cathode is narrowed so that strict control is required during manufacturing processes.